I've looked and looked through my phone, but couldn't work it out either, but I found this on www.drhotdog.net

"First go to Settings > Applications > Manage applications, and tap the Google Apps entry. Then tap 'Clear data', and tap 'OK' to confirm. This will delete your existing Google account details. To enter your new account details go to Settings > Data synchronization > Google and you'll be prompted with the set up dialogue. Enter your new account details and you're done.

What this won't do for you is move all your mail, contacts and calendar data from the old account to the new one, so in order for all your data to appear on your phone as it did before you'll first need to transfer it between the two accounts. I'll describe how I did that in my next post."

So rather than GMail, the app that controls it is Google Apps. Haven't tried it. But maybe worth a try.

Personally though. I would just synch all the contacts, back up my apps to the SDcard and do a factory reset. Then all the links to that old Google Account and data will be resolved. Most apps save their data to the SD card so you shouldn't lose that either.